Abstract Diarrhea currently kills over half a million children under 5 years of age each year. This is partially due to their increased susceptibility to enteric infections. Whereas a fully functional, healthy adult gut microbiota provides ?colonization resistance?, forming a barrier against invading pathogens, the immature microbiota is unable to protect. Development of effective protection correlates with the presence of large numbers of obligate anaerobic species, including the spore-forming Clostridia. Although the phenomenon of colonization resistance has been known for a century, there is a lack of detailed knowledge of the key bacterial species and functional molecules that mediate it. A better basic understanding of this phenomenon is needed in order to prevent enteric infections, as well as intervene in the growing number of diseases that are now linked to the gut microbiota. To address this problem, we will take multiple independent but complementary approaches, built on a strong foundation of preliminary data. In Aim 1, we will fractionate and identify bioactive molecules that are responsible for mediating colonization resistance. In parallel with this, we will use established genetic tools to screen our model pathogen, S. Typhimurium, for genes that affect its susceptibility to colonization resistance. In Aim 2, we will further refine the consortium of protective Clostridia bacteria we previously isolated. We will select small groups and single strains that are able to protect mice, and compare them phenotypically and genetically to find correlates of protection. We will also characterize their growth requirements and nutrient preferences, and test the ability of selected nutrients to support their protective functions in vivo. This will be developed with the eventual goal of creating a prebiotic/probiotic combination to protect susceptible people. At the conclusion of these aims, we will have identified new molecular pathways that control resistance to this model enteric pathogen. We will also have found symbiotic bacteria that are key to protection, and the substrates that support their growth in the developing gut. This will advance our basic knowledge of a critical function of the gut microbiota, and produce new molecules and bacterial strains to develop therapeutically.